use strict;
use warnings;
use 5.010;

my %categories = (
    level1 => {
        'level1-1' => {},
        'level1-2' => {'level1-2-1' => {}},
        'level1-3' => {},
        },
    level2 => {'level2-1' => {}},
    );

dumpHash(\%categories);

sub dumpHash {
    my ($subHash, $indent) = @_;

    $indent //= '';

    given (ref $subHash) {
        when ('') {print "$indent$subHash\n" if defined $subHash}
        when ('HASH') {
            for my $key (sort keys %$subHash) {
                print "$indent$key\n";
                dumpHash($subHash->{$key}, "$indent   ");
            }
        }
    }
}

##</code><code>##

level1
   level1-1
   level1-2
      level1-2-1
   level1-3
level2
   level2-1